1. Field of the Invention
The invention relates to a method of treating water and aqueous systems (hereinafter called the systems to be treated) in pipes with chlorine dioxide (ClO2).
2. Discussion of the Background
Chlorine dioxide can be used in water treatment and for treating aqueous systems because of its high bactericidal, virucidal and algicidal activity. Aqueous systems are used in a multiplicity of industrial processes such as, in the food industry, in brewing processes, in the drink industry and in paper making, inter alia, as transport medium, as heating and cooling medium and for washing purposes. The transport of such aqueous systems typically proceeds primarily in pipes. Generally, biological growth in these systems can be restricted by using biocides such as, chlorine dioxide. Owing to the explosive tendency of gaseous chlorine dioxide (c>300 g/m3) and chlorine dioxide solutions (c>26 g/l), chlorine dioxide cannot be stored in compressed form or in solutions of relatively high concentration. Owing to these chemical properties, chlorine dioxide is typically produced at the point of use. Such point-of-use production is achieved by mixing basic chemicals in special reactors of chlorine dioxide generation systems. The chemical storage vessels, the metering appliances and also the reactor of the chlorine dioxide systems form a locally linked apparatus which is generally erected in rooms accessed by people.
There are a plurality of methods, but principally three underlying methods (see below), for synthesizing ClO2 which have typically been used commercially for water treatment. These methods use sodium chlorite (NaClO2) as one of the starting materials. The underlying chemistry of the three methods is explained hereinafter. The substances used in these methods are referred to as starting chemicals or reactants.
1. Method Using Sodium Chlorite and Strong Acid
In the first method, a strong acid is used together with sodium chlorite. The strong acid is usually hydrochloric acid or sulphuric acid. When hydrochloric acid is used, the reaction stoichiometry is as follows:5NaClO2+4HCl→4ClO2+5NaCl+2H2O.
In addition, chlorine dioxide can be formed with the use of sulphuric acid in accordance with the following reaction:10NaClO2+5H2SO4→8ClO2+5Na2SO4+2HCl+4H2O.2. Method Using Sodium Chlorite and Chlorine
This method uses gaseous chlorine together with sodium chlorite. The reaction proceeds in two stages, first with the formation of hydrochloric acid as follows:Cl2+H2O→HOCl+HCl.Then the intermediate, hypochloric acid (HOCl), reacts with sodium chlorite, forming chlorine dioxide (ClO2) as follows:HOCl+HCl+2NaClO2→2ClO2+2NaCl+H2O.The stoichiometric reaction from the two above equations is:Cl2+2NaClO2→2ClO2+2NaCl.3. Method Using Sodium Chlorite and Sodium Hypochlorite
In the third method, sodium hypochlorite (NaOCl) is used together with sodium chlorite in accordance with the following reactions:NaOCl+HCl→NaCl+HOCl, andHCl+HOCl+2NaClO2→2ClO2+2NaCl+H2O.
The synthesis reactions for generating chlorine dioxide are generally carried out in reactors which are operated either continuously or by the batch method.
Two explosion limits must be taken into account in the generation of chlorine dioxide:                (A) with respect to contact with air, the explosion limit of concern is more than 6 g of ClO2/l of solution, and        (B) with respect to spontaneous autodecomposition, the explosion limit of concern is more than 26 g of ClO2/l of solution.For example, in the case of the chlorine dioxide syntheses carried out by methods 1 to 3, when use is made of starting chemicals which would lead to, in the reaction space, a concentration of greater than approximately 26 g of ClO2/l of solution, dilution water is added to the reaction space in order to bring the concentration below that of spontaneous autodecomposition. When the chlorine dioxide solution leaving the reaction space contains 20 g of ClO2/l or less, which is typical, the solution is diluted with a further water stream to a concentration of roughly less than 3 g of ClO2/l of solution.        
In order for these methods 1 to 3 to be operated with satisfactory results with respect to plant safety, chlorine dioxide yield and time-specific production rate, a variety of processing variations are typically performed:                Installation and use of metering points on the pipe having systems to be treated for addition of chlorine dioxide generated outside the pipe.        Use of diluted starting chemicals such that respective concentrations of the chlorine dioxide solution produced fall below 26 g/l or 6 g/l.        Generation of reduced pressure in the reactor by applying a vacuum in order to obtain reduction of the chlorine dioxide concentration in the gas phase to <300 g/m3.        Generation of reactor overpressure (e.g., by using pressure-retention valves at the reactor outlet) to prevent the formation of a gas phase by exceeding the solubility limit of chlorine dioxide and to increase the yield.        Use of batch methods having long reaction times in order to increase the yield when diluted starting chemicals are used.        Use of superstoichiometric acid amounts in the chlorite/acid method and use of superstoichiometric chlorine amounts in the chlorite/chlorine method to increase the yield.        
Despite the use of the above-mentioned procedures, in the event of incorrect operation of the chlorine dioxide generation systems (e.g., due to loss of dilution water or by failure of the pressure control) spontaneous decomposition (explosion) of chlorine dioxide can occur, or chlorine dioxide may, owing to leakage or breakage of separation surfaces between the chlorine-dioxide-containing solution and the environment, lead to hazards in the surroundings of the generation systems. The use of diluted starting chemicals, which can lead to chlorine dioxide solutions with a concentration of less than 6 g/l and therefore sacrifice the relatively high time-specific generation rates of the chlorine dioxide systems, also cannot prevent the hazard to the surroundings of the generation systems wherein the MAK value (maximum workplace concentration) of 0.1 ppm is exceeded in the event of incorrect operation. In order to minimize these hazards, various measures have been typically implemented at the generation systems themselves, and also at the sites where the chlorine dioxide generation systems are erected. For example, complex servicing work on the generation systems including regular replacement of the reactors, spatially isolating erection sites for the generation systems, and forcing aeration and air monitoring of the atmosphere of the erection site by continuous gas analyses are usually needed.
After production of the chlorine-dioxide-containing solutions, the solutions are generally transported into pipes using pressure elevation appliances. This takes place, for example, via connection ports which are situated in the pipe. The metering line for the chlorine-dioxide-containing solution, which extends into the pipe having the systems to be treated, can only be worked on after clearance of the pipe. Clearance in this case means depressurizing and emptying the system-bearing pipe. The points for chlorine dioxide addition are frequently in bypass lines which are provided with shutoff elements upstream and downstream of the addition site.